Optical heterodyne systems analyze the optical spectrum of an input optical signal under test. A basic optical heterodyne detection system includes an optical coupler that combines an input optical signal from a fiber or device under test with a reference optical signal. The resulting “mixed” optical signal includes a heterodyne “beat” signal at a frequency equal to the frequency difference between the input optical signal and the reference optical signal. The reference signal frequency is changed or “swept” across the bandwidth of the input signal. The detected beat signals over that swept bandwidth are processed to determine one or more characteristics of the input optical signal such as frequency, wavelength, or amplitude.
Typically, heterodyned spectrum analyzers detect amplitude spectra at high resolution compared with other forms of spectrum analyzers. This difference in resolution can be as large as four orders of magnitude. Unfortunately, the local tunable laser source for heterodyned spectrum analyzers is very expensive, and the need for very fine accuracy of the instruments is not clearly shown. As a result, there are very few heterodyned spectrum analyzers on the market.
A problem with conventional optical spectrum analyzers is they only provide amplitude information about the input optical signal's frequency spectrum—phase information is not obtained. But absent that phase information, the input optical signal cannot be accurately reconstructed in the time domain. Indeed, two different signals may have the same amplitude spectrum but different phases. In this situation, a detected amplitude spectrum, by itself, cannot be resolved accurately into one of those two signals.
This difficulty is illustrated in FIGS. 1-5. FIG. 1 shows a binary sequence corresponding to an original time varying signal. The vertical axis is labeled the modulating signal and is graphed against time on the horizontal axis. The original time varying signal modulates an optical “carrier” resulting in a modulated optical signal, which when transmitted over a fiber, may be detected as a frequency spectrum as shown in FIG. 2. One might assume that the original signal could be simply recovered by transforming the frequency spectrum into the time demand, i.e., by applying an inverse Fourier transform to the detected signal. But this assumption is not true. The detected spectrum includes only the amplitude of the signal; the phase is missing. The original time-domain signal cannot be recovered in the inverse Fourier transform operation because the phase information has been lost. The resulting time domain waveform in FIG. 3 resulting from the transform is distorted beyond recognition from what is shown in FIG. 1. Indeed, it is compressed into a single pulse rather than the series of pulses shown in FIG. 1.
If both the phase information as well as the amplitude information can be determined from the frequency spectrum shown in FIG. 2, both the real and imaginary parts of the spectrum could be determined as shown in FIG. 4. Transforming the frequency spectrum of the signal that contains both real and imaginary parts, (both amplitude and phase information), permits recovery of the original information as shown in FIG. 5 which corresponds to the modulating signal information shown in FIG. 1.
An objective of the present invention is to detect an optical signal's complex power spectrum, such as that shown in FIG. 4, and to determine from that detected spectrum both amplitude and phase so that an original information signal can be reconstructed accurately in the time domain. A heterodyne optical signal analyzer in accordance with the present invention performs such a signal reconstruction to accurately recover the full, complex (amplitude and phase) time domain signal as well as the spectrum, thus providing an instrument with greater utility.
All laser transmitters introduce an unwanted phase shift on the optical signal. This unwanted phase shift, referred to as “chirp,” limits the distance over which the optical signal can be sent. The present invention enables precise characterization of the signal chirp permitting accurate prediction of the transmission distance limit, and even how to modify the transmitted optical signal so that longer transmission distances can be achieved.
In addition to resolving both the phase and amplitude of an optical signal as a function of time and chirp characterization, the performance of the heterodyne optical signal analyzer in accordance with the present invention is not limited by the bandwidth of its components. For example, a 40 GHz optical input signal can be accurately processed even though the heterodyne optical signal analyzer operates at a much lower frequency. The heterodyne optical signal analyzer slowly builds a picture of the input signal by sweeping the reference signal below and above the frequency of the input signal to be reconstructed. As a result, signals may be reconstructed with very high resolution. Effective sampling rates as high as 1000 GHz may be achieved. The measurements produced at these rates do not have any detector bandwidths incorporated into them, and thus the signals are nearly free of distortion.
Accordingly, the heterodyne optical signal analyzer in accordance with the present invention permits optical communication system designers to determine in advance what an optical signal will look like when it is transmitted over a fiber network. That information is extremely important to people who design such networks. Currently, no instrument is available that permits one to identify the optical signal being propagated at any point in the network. In other words, the transmitted signal is an unknown. Although the signal's power as a function of time and the amount of chirp (variation in frequency as the power is turned on and off) can be measured, how the amplitude and phase of the signal vary with time is unavailable. Until now, there is no reliable way to predict how a particular transmitter will interact with a specific set of optical components or an overall optical link. Knowing the precise nature of the signals present in a network is the first step in making the network cheaper and more efficient. With conventional optical signal analysis instrumentation, there is a great deal of interpretation and guess-work involved leaving simple analysis problems to highly skilled and expensive people. The invention reduces the problem of analyzing an optical network to an analysis closer to that employed for existing cable or microwave networks.
Other aspects of the present invention relate to calibrating the heterodyne optical signal analyzer. Light in a standard optical fiber includes two orthogonal polarization modes. To fully characterize the light in the heterodyne optical signal analyzer, a vector representation of the light requires determining amplitude and phase for each of the two orthogonal modes. The components of a heterodyne optical signal analyzer, including optical couplers, detector blocks that detect optical power and convert it into an electrical signal, and the reference signal generator, all have errors and offsets. For example, optical power detectors are very sensitive to changes in polarization of the optical input signal and the reference optical signal. Several different calibration procedures, including detector calibration, vector calibration, and reference signal calibration are described below.
In a general example embodiment of the invention, an optical signal analyzer includes a first coupler, a first detector block, and a data processor. The first coupler mixes an optical reference signal and an optical input signal whose characteristics are to be determined and generates multiple mixed signal outputs. The first detector block detects multiple power signals from the multiple mixed signals. Each individual detector in the detector block includes in one example implementation a photodetector, an amplifier, an analog-to-digital converter for converting the amplified output into digital power signal information, and a buffer for storing the digital power signal information.
The data processor determines the original optical input signal in the time domain from those multiple detected power signals. Both the amplitude and the phase of the optical input signal are determined from the detected multiple power signals for each different frequency of the reference signal as it is swept across the frequency bandwidth of the input signal. From these detected outputs over the swept frequency range, the original time domain signal is reconstructed using signal processing procedures outlined below.
In the first example embodiment, the first coupler generates first, second, and third mixed signals, and the first detector block detects corresponding first, second, and third power signals. But in a second example embodiment, a second coupler and a second detector block are added to take into account the two polarizations of light, thereby permitting more accurate signal reconstruction. The first coupler and detector detect mixed signals where the reference signal has a first polarization. The second coupler and detector detect mixed light where the reference signal has a second different polarization. The second coupler generates fourth, fifth and sixth mixed signals, and the second detector detects fourth, fifth and sixth power signals from the fourth, fifth and sixth mixed signals.
In the second example embodiment, the data processor determines a first phasor of the optical input signal using the first, second, and third detected powers and a second phasor of the optical input signal using the fourth, fifth, and sixth detected powers. Using the first and second phasors, the data processor determines the input optical signal in the time domain. In effect, the data processing circuitry determines from the first, second, and third detected powers a first real part and a first imaginary part of the input optical signal in a first complex reference plane. Similarly, from the fourth, fifth, and sixth detected powers, the data processing circuitry determines a second real part and a second imaginary part of the input optical signal in a second complex reference plane. The first complex reference plane corresponds to the first polarization state of the optical reference signal, and the second complex reference plane corresponds to the second polarization state of the optical reference signal. The first real part and the first imaginary part correspond to the first phasor, and the second real part and the second imaginary part correspond to the second phasor.
Together, the first and second phasors accurately represent all the polarization states of the input optical signal and include both the real and imaginary signal components in both polarization states. As a result, the input signal can be reconstructed very accurately in the time domain (more so than when only one phasor is employed).
Although the present invention can be practiced without calibration, better results are achieved when calibration is employed. Typically, optical components will have tolerances that are no better than 5% or 10%. Therefore, if the system is constructed from commonly available optical components, resulting measurements are accurate to this same degree. But higher degrees of accuracy are required. One method of achieving this would be to buy increasingly better optical components, but this approach rapidly becomes cost prohibitive. A better way is to accurately calibrate their unwanted effects on the measured signals, and then computationally remove them from the measurements. In this way, higher accuracy measurements can be achieved at a much lower cost. In fact, the accuracies that can be achieved with this approach may exceed the accuracies that could be achieved regardless of optical component cost.
In a first example calibration, amplitude and phase corrections for the phasor output by the first detector block may be determined and used to generate a phasor calibration matrix for the first detector block. As the reference signal is swept across a range of different frequencies, corresponding detected powers at the first detector block are acquired for each frequency. Each detected power has a high frequency component. Phase differences are calculated between the detected powers in the first detector block using the corresponding acquired high frequency components. The reference signal may again be swept across the range of different wavelengths without an input signal to acquire detected powers in the first detector block. Each detected power has a low frequency component. Amplitude differences between detectors in the first detector block are calculated using the corresponding acquired low frequency components. These phase and amplitude differences are used to generate the phasor calibration matrix. Preferably, the phasor calibration matrix is converted from an arbitrary reference system into complex plane reference system. A similar calibration procedure may be applied to the second detector block in the second example embodiment.
A vector calibration procedure is also desirable in the context of the second example embodiment described above. Accurate vector measurements require that the signal be projected onto two orthogonal vectors with the same length. The measurement is accurate only to the degree that these conditions are satisfied. If those conditions are not satisfied, which they typically are not in a real world system, the system should be calibrated for the non-ideal aspects of the reference vectors. Because optical polarization states within fiber optic networks tend to vary over wavelength, (e.g., as the laser source is tuned), keeping the two reference states perfectly orthogonal is very difficult and expensive. If, however, the two states need only be approximately orthogonal, but repeatable, then the system is much easier to build and only requires a calibration procedure to produce the correct vector measurements.
In the vector calibration procedure, the reference signal is generated at multiple different polarizations, (e.g., four), and the resulting power phasors determined at the first and second detector blocks construct a complex vector corresponding to each different reference signal polarization. A vector calibration matrix is generated using the complex vectors generated for each of the reference signal polarizations. The vector calibration matrix is used in normal operation to convert subsequently detected powers at the first and second detector blocks into an ortho-normal coordinate system.
It is further desirable to calibrate a reference signal generator to ensure that the frequency of the generated reference signal matches the frequency the generator is set to generate. In the frequency calibration procedure, the reference signal is swept across a range of different frequencies. A portion of the reference signal is passed through two different length fiber paths. Light from the two different length paths is detected as a function of wavelength. A reference signal frequency correction is determined using those detected outputs.
A final procedure relates to the recovery of the complex spectrum of the input optical signal. This signal recovery requires the spectrum of the signal to remain constant over the course of the measurement, which is not difficult if the signal is repetitive with some period that is known or can be measured. The processes and calibrations described above allow measurement of the time variation of a complex vector with respect to a known reference field. This raw signal is only a part of the overall signal which may have a bandwidth much larger than the bandwidth of the detector. By observing the signal over a series of bands that completely covers the signal bandwidth, the signal can be reconstructed entirely at a much higher effective sampling rate than would otherwise be possible. The effective sampling rate is determined by the sweep range of the reference laser, and can be on the order of 1000 GHz.
A frequency bandwidth response for each detector block is determined. From the corresponding frequency response, a time domain impulse response of each detector block is determined. The impulse response is used to create a Green's function that relates the input optical signal to be determined as a function of time and the measured signal determined from the detected power as a function of time. The Green's function is inverted and then used to convert the measured signal into the input optical signal.
Other features, aspects, and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Like reference numerals refer to like elements throughout.